Molecular orientation nematic liquid crystals

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

The most straightforward way to produce partially oriented solute molecules is to orient them in anisotropic solvents. Good solute orientation can be achieved in stretched polymer sheets [94, 95, 96]. Homogeneously oriented nematic liquid crystals are perfectly clear and are thus excellently suited as anisotropic solvents for optical polarization experiments. Moreover, the liquid crystal method allows the performance of polarization experiments in fluid media. This unique feature of the liquid crystal method can be exploited for polarization studies of metastable molecular species (e.g. excited complexes) formed by a diffusion-controlled process. The ordered glasses produced by rapid cooling of a uniformly aligned nematic phase can be used for phosphorescence polarization experiments. 

The phenomenon of orientation of liquid crystals by surfaces has been known nearly as long as have liquid crystals themselves [ 1 ]. The phenomenon has mainly been studied in low-molecular-weight nematic liquid crystals, both because of the simplicity of their structure and because of the use of this type of liquid crystal in displays. Most of the present chapter is therefore be dedicated to this type of liquid crystal. 

J. G., De Lange, G. A. Molecular solutes in nematic liquid crystals orientational order and electric field gradients. Chem. Phys. Lett. 1983, 99, 271-274. 

The magnitude of surface tension, y, has also been calculated from statistical theory and molecular orientations at the free surface in nematic liquid crystals. ... 

Liquid crystals are optically anisotropic media with the exception of the cubic phases, such as the D-phase and blue phase. The refractive index or dielectric constant of liquid crystals varies according to the orientation of the molecular axes (or the optical axes). For example, the nematic liquid crystals are optically uniaxial and exhibit remarkable birefringence, that... 

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